1. Field of the Invention
The field of the invention is that of onboard terrain anticollision systems for aircraft.
2. Description of the Prior Art
Collisions with the terrain while the aircraft is fully controlled also called “CFIT”, the acronym standing for “Controlled Flight Into Terrain”, have been and still remain one of the main causes of air disasters. Developed some thirty years ago, the systems termed “GPWS”, the acronym standing for “Ground Proximity Warning System”, have allowed a significant reduction in the number of accidents. They are based on the use of radio-probes which make it possible to determine in an instantaneous manner the position of the aircraft with respect to the ground. These rudimentary and nonpredictive systems have not however made it possible to completely eliminate accidents of this type.
More recently, “GCAS” type systems, the acronym standing for “Ground Avoidance Collision System”, have appeared. These systems rely on the use of systems for predicting potential trajectories of the craft and the determination of possible collisions between these trajectories and the terrain. The pilot can thus anticipate a future collision and react accordingly.
More recently still, terrain anticollision systems have taken the generic term of “TAWS”, the acronym standing for “Terrain Awareness Warning System”, and cover all systems possessing a function for predicting potential collisions with the terrain. These systems are defined by an international aeronautical standard, the TSO C151A, and fulfill in addition to the customary GPWS functions, the additional functions of predictive alert of risks of collision with the relief and/or obstacles on the ground termed “FLTA”, the acronym standing for “predictive Forward-Looking Terrain collision Awareness and alerting” and of premature descent termed “PDA”, the acronym standing for <<Premature Descent Alerting”. These FLTA and PDA functions consist in warning the crew through timely prealerts or alerts whenever, under controlled flight, a situation of risk of collision with the terrain arises, in particular when the short-term foreseeable trajectory of the aircraft encounters the relief and/or an obstacle on the ground, so that an avoidance maneuver is engaged. The pilot can thus avoid the “CFIT” by an appropriate avoidance maneuver. The basic maneuver is termed “pull-up” signifying vertical avoidance.
These functions can according to the implementation be grouped into a single mode termed “CPA”, the acronym standing for “Collision Prediction and Alerting”.
The first generation of “TAWS” systems affords the functions of prediction of potential trajectories, of determination of risk of collision with the terrain, of cartographic display of the terrain comprising the indication of the risk of collision and of audible alerts in the event of risk of collision. Certain second-generation systems of the “TAWS” systems allow not only the prediction of the risk of collision with the terrain, but also alert the pilot as to the feasibility of the disengagement maneuver to be performed to anticipate this risk of collision. This is rendered possible by the use in real time of the upward speed capabilities of the aircraft.
In a more precise manner, the “CPA” mode is based on a comparison between a surface also called the safety profile denoted S or “clearance sensor” and the surface or the terrain profile situated under said surface or said safety profile, said comparison taking account of a safety margin. The terrain profile arises from a topographic representation extracted from a terrain and/or obstacles database onboard the aircraft and is correlated with the position of the aircraft by virtue of the position sensors of the aircraft.
The safety surface or profile S are represented diagrammatically in the two cross sections of FIG. 1 which represent a lateral view and a view from above of said surface or of said profile.
The intersection of said surface S with a vertical plane containing the aircraft A forms a trajectory termed the predicted trajectory TP. In FIG. 1, the origin O of this predicted trajectory is taken under the aircraft, vertical thereto and with a vertical safety margin MV whose value is determined as a function of various parameters such as, for example, the flight phase, the vertical speed of the aircraft, the distance to the closest airport or destination airport. In a first variant of calculation of the “clearance sensor”, the safety margin is in an equivalent manner associated with the profile or with the terrain surface. In this case, the origin O of the trajectory is taken at the level of the aircraft, the origin of the terrain under the aircraft this time being heightened by a vertical safety margin MV. It is possible, of course, to combine in a second variant, the two modes of calculation of the “clearance sensor”, that is to say take the origin O of the trajectory under the aircraft with a first margin and “raise up” the terrain by a second margin, the sum of these two margins being equal to the safety margin MV.
This origin O determined, the predicted trajectory TP comprises two main parts as is indicated in the lateral view of FIG. 1 where the predicted trajectory appears in solid line:                a first part corresponding to a first flight time T1, dependent on a prediction of the trajectory in progress calculated on the basis of the origin O;        a second part corresponding to a second flight time T2 following the first flight time T1, dependent on a prediction of a vertical avoidance trajectory. The first part is calculated on the basis of flight parameters comprising the speed and the roll angles of the aircraft.        
Generally, the flight time T1 is at least equal to the response time necessary for initializing a vertical avoidance maneuver.
The second part is also called “SVRMB”, the acronym standing for “Standard Vertical Recovery Maneuver Boundary”. It models a lower limit of the standard vertical avoidance trajectory supposed to make it possible to avoid the collision with the terrain. The maneuver comprises, for the pilot, the following successive operations:                Reducing the roll angle until horizontal stabilization of the aircraft. By way of example, the rate is 15 degrees per second;        Pulling up the aircraft under a load factor compatible with the performance of the aircraft. By way of example, the load factor is 0.5 g;        Maintaining the pull-up angle of the aircraft either with a standard slope equal to a certain percentage of the possible maximum slope of the aircraft, for example equal to 90 percent, or with a slope equal to the slope of the aircraft when it is already greater than said standard slope. The typical duration T2 of this phase is of the order of 112 seconds. This duration can be modulated as a function of the proximity of the airport for certain mountainous zones or for other flight considerations.        
The future trajectory TF of the aircraft in the event of a vertical avoidance maneuver is depicted dashed in the lateral view of this figure.
The safety surface or profile are limited laterally by a left limit TG and a right limit TD as is seen in the view from above of FIG. 1 where these limits are depicted in solid line. These limits correspond to predicted limit trajectories of the aircraft during a flight duration corresponding to the sum of the first and second flight times T1 and T2, said limits being defined essentially by a lateral margin ML taking its origin at the point O and at least one angle of left lateral aperture θG and at least one angle of right lateral aperture θD, left and right being defined with respect to the sense of the trajectory of the aircraft.
The limits of the terrain situated under the aircraft used for the comparison with the safety surface or profile are obtained by the vertical projection of the left and right limits of the safety surface onto the terrain situated under the aircraft.
The lateral margin ML taken in a horizontal plane passing through the origin O typically equals 100 meters on either side of the aircraft. The aperture angles can vary as a function of the forecast curvature of the future trajectory of the craft in a horizontal plane. Said trajectory is represented dashed in FIG. 1. On the convex side of the curvature, the angle of lateral aperture typically equals a few degrees. It is limited to a value of 90 degrees. On the opposite side, it typically equals 1.5 degrees. In FIG. 1, the aircraft is turning to the right. Consequently, the angle of right lateral aperture θD equals several degrees and the angle of left lateral aperture θG equals 1.5 degrees. In this way, the surface situated between these two limits covers the whole zone capable of being overflown by the aircraft during a flight duration equal to the sum of the first flight time T1 and of the second flight time T2.
Currently, the “CPA” mode calculates two safety surfaces or profiles, the first surface SMT is called the medium-term safety Surface or Profile or else the “Medium Term Clearance Sensor” and the second SCT is called the short-term safety Surface or Profile or else the “Short Term Clearance Sensor”. These surfaces are represented in FIG. 2.
The short-term safety surface or profile are calculated as indicated in the previous paragraphs.
The medium-term safety surface or profile comprise two parts. The first part can be determined in a similar way to the first part of the safety surface or of the short-term profile. The second part corresponds to a second surface or a second safety profile that can be built according to calculation principles similar to those of the short-term safety surface or profile but by taking the origin O′ of said second surface no longer at the level of the aircraft A but on the predicted trajectory ahead of the aircraft. Typically, the first flight time T1 of the predicted trajectory of the medium-term safety surface or profile has a duration of about 20, the first flight time of the predicted trajectory of the short-term safety surface or profile has a duration of about 8 seconds. These values of 20 and of 8 seconds can be modulated as a function of considerations such as the height of the aircraft above the ground, the air-speed of the aircraft, its vertical speed, the proximity of an airport, etc.
The medium-term safety surface or profile SMT is dedicated, in conjunction with the surface or the profile of the terrain, corresponding to the advanced detection of risk of collision with the terrain G as indicated in FIG. 3. The risk of collision is depicted by a white star. In the event of risk of collision, a prealarm is emitted in audible and/or visual form. In this case, the potentially dangerous terrain is depicted typically in plain yellow on the displays of the instrument panel. The pilot can then evaluate the situation and rectify or otherwise his current trajectory.
The short-term safety surface or profile SCT is dedicated, in conjunction with the surface or the profile of the corresponding terrain, to the detection of risk of imminent collision with the terrain G as indicated in FIG. 4. In the event of risk of collision, an alarm is emitted in audible and/or visual form. This alarm is in the general case an alarm termed the vertical avoidance alarm also called “pull-up”. In this case, the dangerous terrain is typically depicted in plain red on the displays of the instrument panel. The pilot absolutely must instigate a vertical avoidance maneuver.
Nevertheless, in certain cases, the alarm associated with a vertical avoidance is replaced by an alarm termed the transverse avoidance alarm also called “avoid terrain”. These cases arise when a vertical avoidance trajectory would not make it possible to avoid the collision with the terrain, typically when starting a turn or stopping a turn in mountainous zones. The transverse avoidance must not, in these particular cases, limit itself to a simple maneuver termed a vertical evade but also integrate a transverse component so as to avoid the collision, the maneuver rate being able to be provided by the TAWS system. In this case, the dangerous terrain is typically represented by alternately red and black bands on the displays of the instrument panel. The pilot absolutely must instigate a transverse avoidance maneuver.
This “avoid terrain” alarm is triggered in certain specific situations detailed below:                When the surface or the profile of the terrain situated in the safety surface or profile exceeds locally at one or more points or in one or more sections in a very significant manner the level of said surface or of said safety profile. In this case, a vertical avoidance maneuver may prove to be insufficient to eliminate any risk of collision. This situation can occur when the craft A is at a significantly lower height than the surrounding terrain, for example, when the craft is in the phase of approach to airports P situated in a mountainous zone as at Calvi, at Chambéry, at Katmandu, at Innsbruck, etc. This case is presented in the lateral and top views of FIG. 5 where the future trajectory TF of the aircraft A is depicted in solid line and the safety surface or profile S is depicted dashed.        When a very wide portion of the surface or of the profile of the terrain enters the safety profile or surface. In this case also, it is not certain that a vertical avoidance maneuver will make it possible to preserve a sufficient vertical safety margin making it possible to avoid the collision. This situation is depicted in FIG. 6.        When the aircraft A changes trajectory rapidly, either by increasing the curvature of its trajectory as indicated in FIG. 7, or by decreasing it as indicated in FIG. 8. In the views of these two figures, the position and the future trajectory TF of the aircraft A have been indicated at the instants T and T+ΔT. At the instant T, the future trajectory symbolized by an arrow did not foretell any collision with the terrain G. At the instant T+ΔT, a change of trajectory gives rise to a risk of near collision.        When the pilot has not reacted sufficiently quickly to a vertical avoidance alarm.        
The transverse avoidance maneuver consists either in carrying out a vertical avoidance maneuver accompanied by a turn with an appropriate radius of deflection, or else in a correction of the last piloting action performed by the pilot to obtain the necessary trajectory correction.
One of the tricky points in the management of “TAWS” systems is to precisely determine the situations in which the transverse avoidance alarm termed “avoid terrain” must be triggered, simple comparison between the medium- and short-term safety surfaces or profiles and the surfaces or terrain profiles possibly proving to be insufficient in the specific situations mentioned above.
Specifically with such a comparison, the height of overshoot of the terrain above the safety surface is not established.